So it turns out, Dalaran Defender’s Treads CAN be transmogged, but they have to be equipped on a class that wears leather. My warrior somehow had leather boots equipped, and you can’t transmog when you’re wearing the wrong armor type.
My DH has the same leather boots on, and they are mogged correctly.
The plate boots are Dalaran Defender’s Boots. Mail gets Stompers. Leather gets Treads. Cloth gets Loafers.
Not sure if that’s why others are experiencing the issue, but that was why I was. They’re working fine for me now.